1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image with a developing liquid.
2. Description of Related Art
A conventional image forming device including a photosensitive drum having a dielectric surface and a unit for supplying to the drum a developing liquid, for example, an insulation carrier liquid, such as aliphatic saturated hydrocarbon with toner particles dispersed therein. An electrostatic image is formed on the dielectric surface of the photosensitive drum. The toner particles in the developing liquid are attracted to electrostatic force of the electrostatic image and selectively adheres to the dielectric surface of the photosensitive drum accordingly. The toner particles are then transferred to a printing medium, thereby forming an image thereon.
The toner particles dispersed in the developing liquid do not need to be as large as the powdered toner particles so that a fine image can be obtained using developing liquid. Also, the developing device using developing liquid requires less energy for fixing toner particles on the printing medium than a device using powdered toner.
However, the aliphatic saturated hydrocarbon serving as the carrier liquid has an unpleasant odor. Therefore, devices using developing liquid are not appropriate for home or office use. In order to overcome this problem, Japanese Patent Publication NO. SHO-52-6091 proposes a developing device using water-based developing liquid. In addition to preventing unpleasant odor, the water-based developing liquid can be prepared using less expensive materials, and so is economical.
However, the device described above has some drawbacks. A developing roller of the device is formed on its outer peripheral surface with a spirally arranged peripheral groove for holding the developing liquid supplied by a developing roller. The pitch of the groove determined the image resolution in the direction extending with the longitudinal axis of the developing roller. That is, the greater pitch of the groove, the lower resolution of the developed image. However, the groove is machined in the developing roller so it is difficult to form the groove with a pitch of 0.1 mm or finer, thereby limiting the resolution. Also, surface tension of the developing liquid prevents clean separation of dots in a direction in which the groove extends. The resultant drops are larger than desired. That is, when a dot's worth of the developing liquid in the groove is selectively attracted to a charged dot region of the photosensitive drum, surface tension will pull liquid around the dot's worth of liquid toward the photosensitive drum with the dot's worth of liquid. As a result, excess developing liquid will adhere to the photosensitive drum, causing poor resolution.